Hormones are prominent regulators of maternal behavior, and there is growing evidence that paternal care is also influenced by endocrine substrates. New World callitrichine primate males are highly involved in rearing offspring; therefore we examined the relationship between paternal behavior and steroid hormones in marmoset (Callithrix geoffroyi) fathers and evaluated the influence of infant care experience on these factors. We measured urinary concentrations of testosterone, estradiol, and cortisol of fathers before and after litter parturition (n=26). It was predicted that there would be a negative relationship between carrying effort and levels of circulating hormone; this relationship was predicted to be influenced by the level of infant care experience. Carrying effort (p<.001), and concentrations of testosterone (p<.001) and cortisol (p=.047) underwent temporal changes over the post-partum period. The patterns of carrying effort (p=.022) and testosterone (p=.024) were influenced by experience level. Primiparous males and males with minimal experience displayed peak levels of carrying effort during post-partum weeks three through six, which is consistent with carrying rates seen in Callithrix kuhlii. Males without experience displayed a significant decrease in testosterone concentration during post-partum weeks three through six (p=.028), while males with experience had low testosterone concentrations throughout the post-partum period. The nature of the relationship between paternal behavior in marmoset (Callithrix geoffroyi) fathers and steroid hormones appears to be influenced by infant care experience.